Cutting centered hole in mirrored object with subsurf
A quad won't give you a real circle anyway. If you look at your example, the outer holes are still bulging a little in the corners. That's due to the algorithm and can't be helped. Six or 8 sides are much more precise. So, since you have 4 (6 with mirroring) verts in the middle, turn it into a hexagon or use 8 to have a nice clean result. Depending on what you want to do with the hole, an additional facering around the holes can be helpful as well as makeing a nice and undistorted bevel later.